This invention relates to a magnetic recording and reproducing device and, more particularly, to a tracking control with improved accuracy during reproduction in a magnetic recording and reproducing device such as an R-DAT (rotary head type digital audio tape recorder) in which a tracking error signal can be obtained only at uneven intervals.
The R-DAT is a device which converts analog signals such as an audio signal into PCM signals, records the PCM signals on a magnetic tape and reproduces the same.
As shown in FIG. 2, the device comprises a rotary head 2 having two magnetic heads A and B separated by 180 degree interval on the circumferential surface of a cylinder 1. A tape 3 is loaded from a cassette housing 4 with a vertical post 5 or with an inclined post 6, wound on the circumferential surface of the rotary head 2 for 90 degrees, supported by a fixed guide 7, and run by a capstan 8 and a pinch roller 9.
The diameter of the rotary head 2 is 30 mm, and the winding angle for the tape is 90 degrees. For recording and reproduction, at the mode I which is usually used, the rate of the rotary head 2 is 2,000 rpm (circumferential speed: 3.14 m/sec) and the speed of the tape 3 is 8.15 mm/sec in the direction identical to that of the rotary head 2. The relative speed of the head 2 as against the tape 3 is 3.13 m/sec.
The recording system with R-DAT is a helical scanning azimuth recording. Its tape format as shown in FIG. 3 defines a track angle of 6.degree. 22' 59.5'' and azimuth angle of .+-.20.degree. with the tracks alternately traced by two heads A and B.
FIG. 4 shows a track format wherein audio data are recorded at the center of a PCM region, and sub-codes and control signals such as ATF (automatic track finding) are recorded on both sides thereof.
The PCM region comprises 128 blocks as shown in FIG. 5 each of which has recording regions for a block synchronizing (indicating the starting position of the block), ID (identification) code, block address, parity check code, and audio data. At the mode I, audio data uses 2's complement codes of quantization bit of 16 bits of the sampling frequency of 48 kHz, and the PCM data is divided into 8 bits in higher order and 8 bits of lower order, modulated from 8 bits into 10 bits (8-10 modulation) and recorded in 10 bits.
For tracking control during reproducing in the R-DAT, an automatic tracking system with ATF is employed. ATF system detects and compares crosstalks from two adjacent tracks by ATF signals recorded on the tracks, and controls the speed of the capstan motor for running the tape so as to make the crosstalks identical.
The principle of ATF will now be described below.
ATF signals are recorded at two locations on one track, i.e., ATF1 and AFT2, as shown in FIG. 4. As shown in the format in FIG. 6, pilot signal f1 and synchronizing signal f2 (or f3) are recorded on each track. The frequencies are specified respectively:
f1=130.67 kHz PA1 f2=522.67 kHz PA1 f3=784.00 kHz
These frequencies are low enough not to have much azimuth loss. The head A traces the track of the synchronizing signals of f2. The head B traces the tracks of the synchronizing signals of f3. The track length of a synchronizing signal differs between an odd number frame and an even number frame, which are defined as one (1) block and 0.5 block respectively.
If it is assumed that the head A is tracing the track T4, pilot signals f1 of adjacent tracks T3 and T5 are obtained from the head A due to crosstalks in addition to a reproduced signal from the track T4. This is because the head has a width 1.5 times as large as a track. If the head A is tracing the track T4 correctly, crosstalks from the tracks T3 and T5 become equal, but if the head is deviated to either direction, the crosstalks from them become different from each other. The crosstalks from the adjacent tracks T5 and T3 are detected by detecting amplitude levels of the pilot signal f1 on the track T5 and of the pilot signal f1 of the track T3 at the detection timing of the synchronizing signal f2 of the track T4. The difference in the crosstalks therefore is presumed to represent tracking errors.
FIG. 7 shows a prior art ATF device based on the above mentioned principle. A reproduced signal from the head A is applied to a low-pass filter 16 via a reproduction amplifier 14 to extract pilot signal f1. The extracted pilot signal f1 is applied to a tracking error detection circuit 21, detected in envelope by an envelope detection circuit 18, and applied to a sample hold circuit 26. A synchronizing signal detector 19 detects synchronizing signal f2 with an equalizer 20 and a comparator 22, and the comparator 22 outputs "1" during the period when the synchronizing signal is being detected.
A logic circuit 24 outputs sample hold signals SP1 and SP2 at the timing of the detected synchronizing signal f2. As the sample hold signal SP1 is provided at a timing immediately after the start of detection of the synchronizing signal f2, if the output from the circuit 18 is sampled with the SP1, a sample hold circuit 26 can hold crosstalk amplitude level of the pilot signal f1 on the track T5 which is the track next to the track T4 currently being traced. A subtractor 28 implements subtraction between the output of the sample hold circuit 26 and the output of the detector 18. A sample hold signal SP2 is produced after the time equivalent to 2 blocks after the start of detection of the synchronizing signals f2 (timing substantially at the center of the pilot signal f1 on the track T3). By sample holding the output from the subtractor 28 with this signal SP2, the difference in crosstalk amplitude levels between the pilot signals f1 of the track T3 and of the track T5 positioned on both sides of the track T4 which is currently being traced is sample held at the circuit 30.
The output from the circuit 30 is applied to a capstan servo circuit 34 as a tracking error signal. The circuit 34 controls the speed of the capstan motor 36 in such a manner that the tracking error becomes zero. This eventually controls the running speed of the tape 10 thereby correcting the tracking error.
In the ATF device shown in FIG. 7, when the head A is deviated to the left from the center, crosstalks of the pilot signal from the track T3 increases. A negative signal is therefore held at the sample hold circuit 30. In order to correct this, the capstan motor 36 increases the speed.
When the head A is deviated to the right, the crosstalk of the pilot signal from the track T5 increases, and a positive signal is held at the circuit 30. For correcting this, the speed of the capstan motor 36 is decreased.
The ATF system can thus control tracking as stated in the foregoing.
In the track format of R-DAT, tracking errors are detected at only two locations, i.e., ATF1 and ATF2 for one track. A tracking error detected, at one detection timing is retained for tracking control until the next time of the tracking error detection. However, the interval of tracking error detection is not uniform as shown in FIG. 8; e.g., in terms of angle, interval from ATF1 to ATF 2 is 64.746.degree. whereas that from ATF2 to ATF1 of the next track is 115.254.degree.. The difference in the interval is about 1 : 2. Therefore, the error signal detected at ATF2 is used for tracking control for a duration of time twice as long as the error signal detected at ATF1. The error signal detected at ATF2 exercises a stronger influence than the one detected at ATF1 with a resulting imbalance in the tracking control.
The head preferably traces a track in parallel thereto as shown in FIG. 9 with a solid line A, but due to errors in the manufacture of the head (uneven circumferential surface of a cylinder or eccentricity of the rotational axis) or warps of the tape due to uneven tension, the head often zig-zags as shown in the figure with a dotted line B or deviates from the center by angle error as shown in a dot-and-chain line C in practice.
In order to correct such defects, there has been proposed a dynamic tracking method which directs a magnetic head with a piezo electric element incorporated in the head so as to force the head to correctly follow the track (for video tape recorder). However, the method has the disadvantage that the structure of the head becomes extremely complicated.
It is therefore necessary to devise a method which allows a head to trace a track optimally without the necessity of addition of such a complicated system even if deviation or zigzagging of the head is unavoidable. The optimal state herein means that the head is placed upon a track at the center of the track width at the center in the longitudinal direction of a track having the audio data PCM region. So far as the head remains in the area, as the width of the head is 1.5 times as large as the width of the track, it can sufficiently read the PCM region to minimize missing data.
If, however, the detection interval is not uniform as mentioned above, such an optimal state cannot be sustained. More specifically, the tracking error signals in the state shown in FIG. 10 become substantially the same in absolute value in ATF1 and ATF2 but with opposite polarities as shown in FIG. 11. Since, however, the tracking servo uses timewise average values of tracking error signals, if the detection interval is not uniform, the average value does not become zero. Tracking error signal in ATF2 therefore exercises a stronger influence. The tracing therefore is controlled in such a manner that the error in ATF2 is corrected to a greater extent, which makes the head to deviate from the center of the track as shown in FIG. 12. Under these state, as the areas where head is deviated from the PCM region becomes enlarged and more PCM data will be missing, making correct reproduction thereof extremely difficult.
It is, therefore, an object of the invention to solve this problem encountered in the prior art and provide a magnetic recording and reproducing device which can conduct tracking in an optimal manner by equalizing effect of tracking error signals when the detection thereof is performed with an uneven interval.